Grey waste water can stink.....worst then black.
Food particles, grease, skin, body oils, soap scum etc.
DON'T leave black OPEN...........grey seems to be a personal choice
Some folks leave grey tank drain valve open until day before then close and let water build up...even add water.
I prefer to leave it closed so that tank gets full and then dump.......that way I know tank is full, don't have to add more water (conserve water usage) and have a full tank that helps carry all the 'stuff' out of tank.
Just like black tank.
I want volume and weight to move the stuff.....
Like a river carrying boulders downstream vs creek bouncing pebbles around :B